Although the term ‘materialism’ has been used in diverse ways in philosophy, it traditionally has had a comparatively precise use within philosophy of mind. In this context, materialism is a certain view, or family of views, in the metaphysics of mind. Specifically, materialism is a certain view, or family of views, on the Mind-Body Problem, which concerns the ontological status of, and fundamental metaphysical relationship between, the mental and the physical—between, for instance, mental properties and physical properties, mental relations and physical relations, mental events and physical events, people and their bodies. (For simplicity, we will hereafter focus primarily on mental and physical properties (and relations); understanding their relationship arguably provides a key to resolving the entire problem.)
Historically, materialism was just the reductionist position that mental properties are identical to—and in that sense are nothing but—physical properties. (Idealism was the competing reductionist answer to the Mind-Body Problem, reducing physical properties to mental properties.) Throughout most of the history of philosophy, materialism took the form of what today we call the Identity Theory, according to which mental properties are identical to internal bodily properties, whether they be the properties associated with Democritean atoms, Hobbesian motions in the body or, in our period, electrochemical interactions at the neurological level. (Of course, nothing prevents such a theory from incorporating environmental factors in order to accommodate content externalism; for us, this kind of extended theory would still count as a materialist ontological reduction.) In the first half of the Twentieth Century another form of materialist reductionism emerged, namely, Behaviorism, according to which mental properties are identical to behavioral properties (dispositions of the body to behave in certain ways in certain circumstances). In the 1960s and ’70s a third form of reductionism gained prominence, namely, functionalism, according to which our standard mental properties and relations (being conscious, thinking, etc.) are identical to (and hence reducible to) second-order properties: specifically, mental properties are held to be definable in terms of the characteristic interactions of their first-order ‘realizer’ properties with one another and the external environment—where in the actual world, and perhaps all possible worlds, these first-order realizer properties are physical properties (presumably, the sort of physical properties invoked by the Identity Theory).2 On a strong version of this view (hereafter called ‘Functionalism’), the realizers of mental properties are necessarily first-order physical properties, from which it follows that mental properties are necessarily second-order physical properties and therefore belong to the general ontological category of physical property. Like the Identity Theory and Behaviorism, Functionalism qualifies as a form of Reductive Materialism.
There is a weaker version of functionalism according to which, even though mental properties are reducible to second-order properties and even though their realizer properties in the actualworld are physical, it is not necessary that the realizer properties be physical. If this view were correct, however,mental properties would not belong to the ontological category of physical property. To see why, consider a world in which the realizer properties are not physical (a possibility implied by this version of functionalism). Plainly, the inhabitants of such a world would be mistaken if they were to assert that mental properties belong to the ontological category of physical property. Therefore, since properties cannot change ontological category, it follows that it would, in the actual world, likewise be a mistake for us to assert that mental properties belong to the ontological category of physical property; on the contrary, mental properties would need to belong to an entirely different ontological category. Given this, this weak version of functionalism does not count as a form of Reductive Materialism, unlike the strong version described in the previous paragraph. There is another weak version of functionalism that is like this one except that it simply remains neutral on the question of whether it is necessary or contingent that the first-order realizers of mental properties be physical properties. This version does not on its own count as a form of Reductive Materialism (but only in conjunction with the independent thesis that it is necessary that the first-order realizers of mental properties be physical). We will call these two weaker versions of functionalism Minimal Second-order Functionalism, or Minimal Functionalism for short.
Footnote 2: David Lewis construes his functionalism as a form of first-order Identity Theory. This construal is dependent on his implausible view that our paradigmatic mental expressions are nonrigid designators of mental properties and relations. This view of these expressions fails for all of our core mental verbs and verb phrases: ‘thinks’, ‘believes’, ‘perceives’, ‘experiences’, ‘senses’, ‘feels’, ‘is aware of’, ‘is conscious of’, etc. By applying the operation of relation abstraction to these expressions, we get the following relation-abstracts: ‘the relation of thinking’, ‘the relation of believing’, etc. Such expressions are rigid designators, as Lewis himself acknowledges, and they denote core mental relations (the relation of thinking, the relation of believing, etc.). Analogously for verb phrases such as ‘thinks that human beings exist’: the associated property abstract ‘the property of thinking that human beings exist’ rigidly denotes the property of thinking that human beings exist. Expressions like ‘pain’, by contrast, do not even denote properties. On two core uses of the expression ‘pain’ (the core uses, we believe), ‘pain’ functions as a count noun which applies to pains, and it also functions as an associated mass noun for more or less pain (more or less in intensity or extent) or for some pain (some amount of pain). The mental property associated with the count-noun use is the sortal property of being a pain, and the mental property associated with the mass-noun use is the property of being some pain. The associated property-abstracts ‘being a pain’ and ‘being some pain’ are rigid designators of these properties. On Lewis’s functionalism, therefore, all of the above mental properties and relations (the property of being a pain, the property of thinking that human beings exist, the relation of thinking, etc.) are rigidly designated second-order properties and relations. That is, Lewis’s functionalism is just another instance of functionalism, as it was characterized in the text.